N. meningitidis is a non-motile, Gram-negative diplococcus that is pathogenic in humans.
Based on the organism's capsular polysaccharide, 12 serogroups of N. meningitidis have been identified. Group A is the pathogen most often implicated in epidemic disease in sub-Saharan Africa. Serogroups B and C are responsible for the vast majority of cases in the United States and in most developed countries. Serogroups W135 and Y are responsible for the rest of the cases in the United States and developed countries.
The meningococcal vaccine currently in use is a tetravalent polysaccharide vaccine composed of serogroups A, C, Y and W135. Meningococcus B remains a problem, however. The polysaccharide approach cannot be used because the menB capsular polysaccharide is a polymer of α(2-8)-linked N-acetyl neuraminic acid that is also present in mammalian tissue. One approach to a menB vaccine uses mixtures of outer membrane proteins (OMPs). To overcome the antigenic variability, multivalent vaccines containing up to nine different porins have been constructed [e.g., Poolman J T (1992) Development of a meningococcal vaccine. Infect. Agents Dis. 4:13-28]. Additional proteins to be used in outer membrane vaccines have been the opa and opc proteins, but none of these approaches have been able to overcome the antigenic variability [e.g., Ala'Aldeen & Borriello (1996)]. The meningococcal transferrin-binding proteins 1 and 2 are both surface exposed and generate bactericidal antibodies capable of killing homologous and heterologous strains. [Vaccine 14(1):49-53].